Where does the power to the wheel spin sensors come from? My XJ8 (1998) has no voltage to the jumper wire for left front sensor. All other tests look good, but the dash ABS, Traction & Stability control, and Cruise have all failed at once. Where does this low voltage originate?

 

They may not be powered. (The moving reluctor can induce a voltage.)

 

According to the manual, there should be a static voltage of approx.
2.4 vdc. My other 3 checked out at 2.39 to 2.4. If this input voltage with acc. key "on" is not reading, the sensor loop back to the ecm isn't complete, I think.

 

desertdawg, looking at the diagrams for your car, the only place the voltage can come from is the ABS module as this is where the wheel speed sensors feed into. So, you have 1 of 2 issues: 1) failure of the ABS module, or broken wire between the ABS module and the speed sensor. To determine which it is, you can either remove the connector at the ABS module and short the 2 pins together (pins 17 and 18) and then see if you have continuity at the sensor or you can get yourself a signal tracer unit (they are like $20 on e-bay) and then you inject a signal on one of the wires and see if you get it all the way up to the ABS unit and then inject it on the other. If you get the signal all the way up to the ABS unit on both wires, then you know the ABS unit is bad. if you get it only on 1 of the 2 wires, then you know which wire is bad and now you need to figure out where the wire is broken. You can run the tracer along the wire and where the signal stops is really close to where the problem lies.

 

I've moved your question from General Tech Help to X308 forum. This is the place to post technical questions about your model.

Graham

 

The wheel speed sensors generate there own power '( but is not the power that is important ) or the signal to the module

if you spin the wheel in the correct direction you will see around 1 volts as a AC signal

this AC signal s how the module counts the AC signal wave heights to determain the wheels rotation speed

It is easier the read this signal at the module connector

Are the sensor " faces " clean as this is the first thing to do

 

Parker, I can see where some of the early units may have needed power as they were using a DC source of voltage to generate a field and then let the wheel interrupt this field, inducing an AC voltage on top of the DC voltage. This would get the speed signal up. Manufacturers quickly figured out that you can make things much more simple by just installing a small magnet inside the coil and this would work the same way. You are using the permanent magnetic field to be interrupted/moved to cause the magnetic field to move, inducing the AC voltage.

 

As a fundamental meter reading through the inductor wheel speed sensors they were 1300 ohms but have heard of 600 on later X300s and that may be the same case on the X308

There was a 4 sensor set part # change during the X300 production

Someone recently was able to modify the X308 to work on the X300 because the rear axles are different in a small way

A functional reading requires a scope but if you spin the wheel in the correct direction you should see around 1.0 volts AC ( but then this is voltage force and not frequency )

 

Parker 7, the voltage induced is a function of a few things. The big one being the speed at which the magnetic field is moved (or the speed of the passing metal object as all other parts are stationary). You then have the strength of the magnetic field. The stronger the field, the more voltage that will get created. Finally you have the size (conductance) of the metal object (this is a function of the type of metal, shape, and size). So, at slow speeds, you may have say a volt coming out of the sensor. But, up at highway speeds, you may have a few volts. You normally start getting limited by the characteristics of the coil, but that is a complete different story in itself.

The silly stuff you learn on submarines when you are bored and start reading the tech manuals as that is the only reading material that you are allowed to have. It was that or clean. Hmmmm, hard decision there.

 

I almost went to Annapolis in the Nuclear Propulsion Officer's Program but I could see myself on a submarine for 6 years after commission

Went to Chemical Engineering School and had a career as a Jet Doctor in the airlines

One thing I don't understand is a left sensor will not work on the right ( from reading post of others ) as the installation would have the sensor face see the reluctor run backwards

The phase angle in relation ship to the reluctor teeth to the face of the sensor face may be different but the total signal sweep should be the same

What I was able to observe was about 1 volt AC with the wheel spinning by hand when jacked off the ground

I didn't spin the wheel in the reverse travel or opposite direction

 

Parker 7, ha ha ha ha. You and I could have served on the same ship. Granted, I would have been enlisted. I did nuclear in the navy for 20 years. I wonder why people still call me Homer though. D'oh.

As for why the sensors are not interchangeable from side to side may be more to do with interference than actual functionality. But, not knowing all the details, hard to say.

As for the voltage, next time you decide to play with the sensors, have some fun and learn some. Try spinning the wheel slow and as fast as you can spin it. See the voltage difference. I have an O'scope and may play with my car some day when I have things torn down (am looking at doing brakes here soon). May be an interesting tidbit of information to learn.

 

There is no difference wich way you spin the tire to read wheel speed sensor signal. Not in cars in this era. Sinewave phase is different between the wheel direction. But that does not make any difference here. Depending on a abs module there might be some ciruittry test voltage in sensor wires. But it has no fuction to the signal. Not with sensors that has a magnet. Abs module reads signal peak to peak time and compares it to the constant tick tock inside the module.

More modern stuff sensors with tiny magnets as trigger wheel is whole another thing. These sensors too has two pins..